whole system freeze when left running - 3.2.8, 4.0.0, and 4.0.4

Description

VirtualBox appears to be freezing my system when left running for more than a few minutes. It doesn't happen quickly but every time I have it running for at least 20 minutes or so (I haven't kept track of the actual time) I will be confronted with a completely frozen computer. This has happened on a 17" i7 MBP, and a 15" Core2duo MBP running 10.6.5 and 10.6.6.

In version 3.2.8 I would get kernel panics whether I was running VirtualBox or not, and in version 4 the system just freezes in place while I'm running VirtualBox. Most of the time this has happened when I let the computers sit idle for some time. A couple times it has happened while I was actively doing something in the host OS. I don't recall a freeze yet happening if I was actively clicking around in the guest OS.